A known air cleaner for an internal combustion engine is provided with a cyclone dust collection unit and a filter dust collection unit that are connected in series in such a manner that dust with relatively large particle diameters are removed from the intake air in the cyclone dust collection unit and the intake air expelled from the cyclone dust collection unit is then forwarded to the filter dust collection unit to have dust of relatively small particle diameters removed from the forwarded intake air. See JPS61-108861A, for instance. According to this arrangement, the clogging of the filter element in the filter dust collection unit is reduced, and the interval of replacing the filter element can be thereby extended.
However, in the arrangement disclosed in JPS61-108861A, the cyclone dust collection unit having a relatively large size protrudes sideways from the filter dust collection unit, and the lateral dimension of the air cleaner is undesirably increased.